OAuth2 linking to existing 2FA enabled account fails

[willemvd]

• Gitea version (or commit ref): master
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant

Mentioned by @geek1011 at the gitter channel and confirmed by myself on try.gitea.io

If you have setup an existing account with 2FA enabled and want to sign in with Github it fails to link the Github account to the existing account.

Steps to reproduce:

• Setup an local account and enable 2FA
• Logout the account and sign in with Github
• On the sign-in-or-create-account page login with the existing local account with 2FA
• Login with the 2FA code
• Your Settings - External Accounts -> here should a Github entry be mentioned but isn't
• Logout and try to login with Github again
• The sign-in-or-create-account page is shown again instead of the user being logged in